This invention relates to a method in which short-pulse laser light having an absorption wavelength matching the optical characteristics of a substance to be processed or measured is used to process the surface or interior of the substance with high efficiency and precision but without producing any thermal effects, cracks and other unwanted phenomena in the substance. Typically, this invention finds specific use in the surface or thin-film processing technology, or in the technology of processing the surface or interior of transparent substances such as glass, or in medical technology for examining the sub-surface area of living tissues.
In the invention, short-pulse Raman laser light is used as short-pulse laser light and this offers an advantage in that the wavelength or pulse duration which match laser light absorption by the substance of interest can be varied easily and in a compact way such as by varying the Raman medium, changing the Raman pump laser wavelength, or using harmonics of Raman light.
In the invention, laser light is concentrated and condensed as much as possible to provide high enough luminance in processing and measurement. In order to ensure that light is concentrated on the substance of interest with high enough intensity, the quality of laser beam has conventionally been improved to achieve spatial concentration. In the present invention, the degree of laser light's concentration is enhanced as an overall value that is expressed by (temporal concentration)×(spatial concentration)×(wavelength-related concentration). Temporal concentration is achieved by short pulses whose duration ranges from the sub-pico to pico second order; spatial concentration is achieved by the good spatial mode of Raman light; and wavelength-related concentration is achieved by allowing the substance to effect one-photon or many-photon absorption of the light in a nearly resonant manner.
By adopting this technique, the heat input to the substance of interest is limited to the smallest possible level so that efficient processing and measurement are realized by means of small laser equipment. The performance of the short-pulse Raman laser satisfies all these requirements.
Before the development of the short-pulse Raman laser, no lasers were available that satisfied the requirements for short pulse duration, high peak intensity, high repetition rate and wavelength tunability and which still were compact and capable of being used to process substances in various industrial fields. In addition, the conventional lasers have had the following problems that prevent their effective use in measurement, processing or medical treatment by adjusting optimum laser wavelength or pulse duration for the substance of interest so that no significant thermal effects will be left on the substance.
(1) In the common CPA (chirped pulse amplification) system, the laser or active medium has a broad wavelength range but the system is bulky and complex, making it necessary to precisely adjust the pulse waveform, spatial mode, polarization, dispersion and other factors of laser beam. As a result, the wavelength and pulse duration of laser light are difficult to alter in a desired way and prolonged laser operation is also difficult.
(2) In OPA (optical parametric amplification), amplification is achieved by white, short-pulse laser light from a wavelength-tunable short-pulse laser; however, it generates only insufficient pulse energy to be useful in processing, measurement and medical treatment.
Hence, the effort to apply the conventional lasers to short-pulse laser processing has involved the following problems. If processing is performed with an amplified T1 sapphire laser, adequate intensity is obtained but due to the limited oscillating frequency, it is difficult to achieve resonance, or frequency match with the absorption band for the substance of interest. If there is no match with the absorption band, processing requires far more intense pulses than are required when resonance is achieved and the leading edge of a pulse first generates an intense Stark spread in the substance and an ensuing pulse causes electrons to be driven by the electric field of the laser.
As a result, many-photon absorption occurs to create a high-energy state and a multi-valent ion plasma develops to break down the substance by the coulomb force, leading to ablation. If the energy of the absorption band is small (i.e. the wavelength is long), the absorption of a single photon is sufficient to achieve processing as expressed by the following relations:ΔE˜hν(one photon provides near resonance) ΔE≠hν(one photon provides no resonance) where ΔE is the energy absorbed by the substance of interest, ν is the number of vibrations of one laser photon, and h is Planck's constant (hν: the energy of a photon).
However, if the energy of the absorption band is large (i.e. the wavelength is short), many photons have to be absorbed to achieve processing as expressed by the following relations:ΔE=nhν(n photons provide near resonance) ΔE=nhν(n photons provide no resonance) 
In the many-photon absorption process, if the energy of the absorption band for the substance differs greatly from the many-photon wavelength of laser (nhν), the processing reaction first takes place at the point in time when laser light having an extremely high trailing edge of pulse is launched and the leading edge of the laser pulse is not used effectively. This is a serious problem since the laser light has only short pulse duration.
As just described above, if there is a great wavelength mismatch, the interaction with the substance of interest starts from the tail of the intense Stark spread at the middle stage of pulse illumination and the leading edge of the short pulse does not make any contribution to processing; this not only results in inefficient use of the laser light but also leads to the development of cracks in the substance due to unwanted light; if the processing does not cause significant thermal adverse effects, lossy heating of the substance occurs.
The only laser that can be used for measurement and processing purposes without experiencing the problems described above is the short-pulse Raman laser which has the following features.
(1) An optimum pulse duration (of the pico to sub-pico second order) that is necessary for processing can be greatly changed by simply altering the conditions for condensing light from the pump laser or the type of crystal. Raman laser pulses are generated if light is condensed to provide a shape (length) exceeding the Raman threshold and, hence, the pulse duration can be changed in an easy and compact way by adjusting focal length of the lens or the beam in the Raman medium.
(2) Wavelength can be tuned in an easy and compact way by using the following techniques either alone or in combination:                changing the type of Raman crystal to choose a suitable wavelength and pulse duration;        choosing the wavelength of the pump laser        performing nonlinear transformations of Raman laser light (i.e. generating harmonics which are one half, a third, a fourth or otherwise of the initial wavelength, adding the energies of any two light beams to generate the sum frequency, or subtracting the energy of any one light beam from the energy of another light beam to generate the difference frequency), thereby permitting wavelength choice from a broad range.        
(3) When performing these nonlinear transformations, Raman laser light can be obtained coaxially with the pump beam, so adjustments such as the change of Raman crystal can be easily accomplished.
(4) Raman light is generated by the nonlinear effect that occurs in areas of high optical intensity and, unlike the conventional laser medium, the Raman medium is not heated, so the desired wave plane is automatically obtained; in addition, the high-quality light produced in the oscillator is amplified by the Raman laser amplifier, so high-quality light having an intensity of at least TW/cm2 which is necessary for non-thermal processing can be easily attained.
The present invention has been accomplished in order to enable the following operations to be performed in the laser-based processing or measuring technology.                (1) high-efficiency, non-thermal, fast processing or measurement using a short-pulse laser having an optimum wavelength and pulse duration for the substance of interest (this can be accomplished by a short-pulse Raman laser);        (2) processing by ultra-fine condensation of light that can be realized by the Raman laser featuring flat wave planes;        (3) micro-processing that can realize finer condensation of light than achieved by the apparent beam size since the many-photon effect is more pronounced in the center of the beam than in the other areas.        
In order to attain these objects, the present inventors gathered the data described below and developed optimum techniques for spectral measurement and processing.
(1) It has been found that the processing steps evolved in the following way as the result of changing parameters such as laser intensity and pulse duration. The processing steps depend largely upon many factors including pulse duration, energy and wavelength of the laser, as well as the heat conductivity of the material.
1) Arrival of Laser Light to the Surface of the Work
The leading edge of laser pulse is not yet intense enough but upon arrival to the surface of the substance, polarization is induced on the surface of the material or surface electrons, being sensitive to the electric field of the laser, come into vigorous motion. In the absence of resonant absorption, surface reflection occurs.
2) Start of Absorption
If the material has a resonant absorption band, electrons in the outermost shell become resonantly sensitive and come into motion; the moving electrons are perturbed by the electric field of the laser which progressively grows in intensity and the process of ionization continues as the electrons repeatedly impinge on the surrounding atoms. If a wavelength is used that does not match resonant absorption, electrons in an outer shell start to move only after the pulse has acquired a strong enough intensity; in other words, the leading edge of the laser pulse is not effectively absorbed and it is necessary to use pulses having a stronger overall energy.
3) Heating in the Direction of Travel of Laser Light
The short-pulse laser has a pulse duration of the sub-pico to pico second order and applies heat by condensing the laser light over a length of the 100 μm order. As a result, compared to conventional laser working with pulse durations longer than the nano-second order, the short-pulse laser allows for the progress of sufficiently local heating to minimize the thermal effect on areas that should be left intact.
4) Effect on Electrons in Inner Shells
As its intensity increases, the influence of the applied electric field shifts from the electrons in the outermost shell to those in inner shells. Upon the first occurrence of resonant absorption, electrons are stripped off and the electric field permeates further inward. However, in the absence of resonant absorption, the laser must have more intense electric field in order to remove the electronic shield.
5) End of Electronic Drive
The laser pulses drive electrons such that they are accelerated to go further beyond the areas illuminated with the laser beam. The degree of electron's acceleration depends on the energy of the laser pulses. When the application of the laser pulses ends, thee is no applied electric field that drives electrons.
6) Heating in a Direction Perpendicular to the Direction of Laser Launching.
The energy of an electron is transferred to the lattice within a very short period of the pico second order, whereupon the temperature of the material begins to rise. When the temperature increases to the melting point of the material, the material starts to melt and when its boiling point is reached, the material evaporates, whereupon further impingement is repeated to create a plasma. The electrons accelerated into the solid undergo further impingement and not only the areas illuminated with the laser but also other areas are thermally affected. Hence, in the absence of resonant absorption, an even higher pulse energy is applied and electrons are further accelerated by the applied electric field of the laser to expand the area being heated. In this way, thermally affected areas occur around the area to be worked and this in turn contributes to the development of mechanical cracks in areas surrounding those thermally affected areas. An effective way to reduce these adversely affected areas is by concentrating the laser energy in an amount that is minimum and necessary for working the material.
7) Plasma Diffusion and Heating
High-energy electrons within the plasma impinge against atoms as they diffuse and this triggers another process of ionization. If the progress of plasma diffusion has reached a sufficient stage, cooling occurs adiabatically and recombination also occurs to effect recovery to the initial neutral atoms.
8) Cooling of Interior of the Solid
Sufficient temperature diffusion through the solid promotes the cooling process.
(2) Presence of Resonance Effect as Compared with its Absence.
Comparison is now made for the following two substances.                A) a substance in which effective absorption takes place due, for example, to the many-photon effect        B) a substance in which absorption does not take place easily in the absence of the many-photon effect        
Considering the processing steps 1)-8), the substance A) is capable of more rapid energy absorption than B) and the laser energy is used more effectively to process the illuminated areas. On the other hand, the substance B) allows the internal electrons to be accelerated to areas beyond the areas to be worked. Therefore, it would be effective to use a wavelength-tunable laser that generates short pulses at a wavelength that can be efficiently absorbed by the material. Needless to say, the effect will become very pronounced if direct processing is performed with the short-wavelength laser. It should, however, be noted that in the absence of many-photon absorption, the surface and the interior of the material are similarly processed and it is impossible to achieve selective processing of the interior of a transparent material.
(3) Choice of Raman Medium and Generation of Harmonics for Realizing Optimum Wavelength (see Table 1)
(4) Condensing Raman Pump Laser for Realizing Optimum Pulse Duration
In the present invention, the following measures are taken to make the most of the above-described features of the short-pulse Raman laser.
1) Changing the Pulse Duration
FIG. 1 shows in the top a conventional short-pulse laser system and it shows in the bottom a Raman laser (crystal) and the theory of its oscillation. Raman laser light 1 has such a pulse duration that if the pump laser light 2 from the conventional short-pulse laser system is condensed in a Raman crystal 3, Raman light is generated within the crystal in an area exceeding the Raman threshold value and amplification occurs as backscattering (backward Raman light 4). The shortest pulse duration is limited by the phonon life. Hence, use of a short-focus optical system or a crystal of short phonon life helps generate short-pulse Raman laser light. Of course, the shortest-pulse laser cannot be made shorter than the phonon life which is inherent in the Raman crystal but longer pulses can be determined by adjusting the length of the optical system or the crystal.
2) Choice of Optimum Absorption Wavelength
i) In the Case of Using the Single-photon Absorption Process
The single-photon absorption process represents the case where the wavelength of the laser is in direct agreement with the absorption band (energy difference ΔE) for the material of interest. Upon illumination with this light, the material first starts to absorb it. If the transition energy of the material matches the wavelength of the light, resonant energy transfer to electrons can be accomplished. If a high-intensity laser field is applied such that a wavelength mismatch causes a corresponding quantity of the Stark spread to be produced, energy can be transferred to electrons after the necessary intensity is reached. If there is a shift from the resonance energy in a low-intensity laser field, energy transfer is initiated in response to a slight absorption of the tail of the Stark spread in the material.
If a large quantity of energy cannot be transferred to electrons in the material at the first stage of the process, the short-pulse laser has difficulty in achieving effective processing on account of the short pulse duration. This is why absorption of light at optimum wavelength is critical. Since the short-pulse Raman laser can utilize Raman light, various wavelengths can be chosen by selecting a suitable Raman medium.
ii) In the Case of Using the Multi-photon Absorption Process
The multi-photon absorption process is the process of absorbing two or more photons of the same origin. For the sake of convenience in explanation, let us assume the case of using an atom as a target of illumination. If the atom makes a transition from the (γJ) state to the (γ′J′) state and if the transition matrix is written as D, the atom's dipole moment μ is expressed by the following equation:μ=<γJ|D|γ′J′>
The intensity of the electric field ε is given by:ε=(2 πφ/c)½where φ is the flux of photons.
The AC Stark spread Ω is written as:Ω=εnΣ1 . . . Σn[nμ1,2 . . . μn,n−1/(E−E1+hω). . . (E−En−1+nhω)]where n is the number of actual energy levels involved, and μ is the matrix of transition between n actual energy levels.
If the amount of this shift becomes comparable to the mismatch in the energy transitional value, a strong transition occurs. Such a great shift can be caused in a laser field having high intensity level of ε. A solid involves a field-dependent Stark spread and requires more sophisticated consideration than the above-described single molecular or atomic model; however, estimation is essentially the same between the two cases.
From the above equation, one can see that excessive laser light must be used if there is a great mismatch; by choosing the resonant wavelength, a process can be realized that is efficient and will not impose any undue thermal impact on the substance of interest.
The actual substance is complex and intricately dependent on illumination and other factors. To explain analytically, let us consider the simplest case of a one-photon transition from an isolated atom. The density N of the upper energy level is expressed by:N=(N0 ΩR2/(Δ2+ΩR2)) (1−cos(Δ2+ΩR2) ½t)/2 where N0 is the total number of the photons, ΩR=2εD/h or Rabi frequency, ε is the intensity of electric field, D is dipole moment, Δ is detuning (the mismatch between the resonance energy and the laser excitation energy).
In order to increase the value of N, Δ has to be increased to a value comparable to ΩR. Conversely, if Δ is large, the laser output has to be increased to increase the Rabi frequency. With large dipole moments as in atoms, ΩR is about 10 MHz at an illumination intensity of about 1 W/cm2. Conventional lasers emit intensities of GW/cm2 whereas the short-pulse Raman laser of the present invention emits an intensity of the TW/cm2 order. In other words, the Rabi frequency at the intensity of the conventional lasers is about 100 GHz whereas the short-pulse Raman laser has a Rabi frequency of about 100 THz. This means that the short-pulse Raman laser has a greater chance of causing multi-photon transition. The relative occurrence of this transition is roughly calculated below using the coefficient ΩR2/(Δ2+ΩR2) and compared with experimetal values.
Assume the processing of glass having the spectrum shown in FIG. 2. A conventional laser (YAG laser) emits at a wavelength of 1060 nm and the short-pulse Raman laser to be used in the invention emits at a wavelength of 1200 nm. The gap energy is approximately 4 eV (≈300 nm). In the conventional laser, three photons are sufficient to gain a value near the gap energy, so the detuning Δ is calculated as follows: 300 nm−1060 nm/3=1000 THz−852 THz=148 THz. In the short-pulse Raman laser, Δ=300 nm−1200 nm/4=0 (resonance). This is the condition for resonance to occur in the multi-photon transition process.
Hence, the value of ΩR2/(Δ2+ΩR2) is very small (4.5×10−6) with the conventional laser but approximates unity with the short-pulse Raman laser. These are theoretical values calculated for maximum intensity and when the laser produces low output in the initial pulse period, the difference becomes greater. Thus, the oscillating wavelength of the short-pulse Raman laser coincides with the multi-photon absorption wavelength and its laser intensity is high; as a result, even four photons which are less efficient than the three photons emitted from the conventional laser can instantaneously be excited 100% to the upper energy level.
iii) Changing the Raman Laser Wavelength and the Shortest Pulse Duration
The Raman laser wavelength and the pulse duration can be altered by using Raman crystals made of the Raman materials listed in Table 1.
TABLE 1SurfaceLineGain co-PhonondamageRamanShiftwidthefficientlifethresholdmaterial(1/cm)(1/cm)(cm/GW)(ps)(GW/cm2)StateStateBa(NO3) 21048.60.410252.5(lns)solidsolidNaNO31069.27.010solidsolidCaCO31086.41.21.68.3 10(lns)solidsolidKGd(WO4) 29015.43.81.9solidsolid7686.44.41.67571.00.7LiNbO3256238.9(2)0.42.5solidsolid637209.4LiTaO3201224.4solidsolidH2(50atm)41551.070gasgas3) System Examples
Processing by illumination with the short-pulse Raman laser requires an intensity of the TW/cm2 order and it is necessary to adjust this value to the specific substance to be processed or measured in industry. A suitable configuration of the short-pulse Raman laser is shown in FIG. 3.
The system of the present invention permits the use of a pump laser having various pulse durations and wavelengths. A chosen Raman medium is illuminated with this pump laser to generate a first-stage short-pulse Raman laser having the fundamental wave and the first through fourth harmonics at various wavelengths (with pulse durations of 20-40 ps). A chosen Raman medium is illuminated with this first-stage laser to generate a second-stage short-pulse Raman laser having the fundamental wave and the first through fourth harmonics at various wavelengths (with pulse durations of 1-4 ps).